PERTEMUAN 14 PENGUKURAN ENERGI

Measurement of Energy Energy is measured by means of the energy meter (watt-hour- meter) Energy meter is an integrating instrument and takes into account both of the quantities Three types of energy meter : –Electrolytic meters –Motor meters –Clock meters

Electrolytic Meters Operates on the principle of electrolytic action and is purely a dc meter measuring ampere-hour Used for measurement of energy when the supply voltage is constant Advantages : –They are accurate even at very small loads –They are unaffected by stray magnetic fields –They are free from friction loads Disadvantages : –There is a potential drop across their terminals –They require frequent inspection –The destruction of the old record of energy on re-set –It is not taking account the variations of circuit voltage

Reason Electrolytic Meters It is generally used on d.c. supply systems The range of the meter is extended by providing it with shunt No appreciable error is introduced at low loads due to any departure from the correct division of current between meter ans shunt

Types of Motor Meters Principle : small motor of d.c. or a.c. type whose instantaneous speed of rotation is proportional to the circuit current Essential parts : –Operating torque system –Braking device –A device for registering the number of revolutions made by rotating elements Mechanical construction : must be such that the functioning of the meter is not readily interfered with by tilting or any external means. Errors in motor meters : friction errors and braking errors With the increase in temperature, the resistance of eddy current path will increase and therefore the braking torque will decrease

Mercury Motor Meters Ferranti mercury motor meter -  It is the most common used and best form of mercury ampere hour meter Illustration of principle of Ferranti D.C. A-H meter  There is a lack of strict proportionality between the flux in the magnet and the voltage creating it

Commutator Motor Meters It is essentially small d.c. motor with a wound armature Advantages : –Simple construction –Small voltage drop across the meter –The considerably large current carrying capacity without use of shunt –Small starting friction owing to the very small pressure on the bearings

Motor Meters for A.C. Circuits The most commonly used meter is the induction type watthour meter Advantage : higher torque/weight ratio and the absence of commutator --  more accurate than the commutator type on the light loads Principle of operation : exactly the same as that of induction type wattmeter The construction is also the same except that the spring control and pointer od wattmeter are replaced by a brake magnet

Induction Type Single Phase Watt-hour Meters Errors and adjustments : –Phase and speed errors : bringing the shading band nearer to the disc, adjustment of the position of the brake magnet –Friction compensation : variation of the positions of the bands –Temperature and frequency errors: They can be adjusted to have a minimum error at declared supply frequency